(a) Field of the Invention
The present invention relates to a method of estimating carrier frequency offset in an OFDM (orthogonal frequency division multiplexing) system. More particularly, the present invention relates to a method of estimating carrier frequency offset in an OFDM system in which a repetitive signal structure inside of a 1 OFDM symbol is used to enlarge the range and increase the accuracy of offset estimation.
(b) Description of the Related Art
OFDM systems are used for high-speed data transmission. In the OFDM system, input bits are converted to data symbols (Xk), and the data symbols are modulated to OFDM symbols through an OFDM modulator using IFFT (inverse fast Fourier transform), and then transmitted to a receiver. At this time, the OFDM symbols are comprised of an N samples and a cyclic prefix, which is a copy of the last part of the OFDM symbol. The cyclic prefix is introduced to maintain orthogonality between sub-carriers, and is, in general, assumed to be longer than the impulse response of a time-dispersive channel.
In the receiver, after compensating for the carrier frequency offset of the received signals and removing the cyclic prefix, a FFT is executed and the data symbols are demodulated.
An offset estimation method of carrier frequency proposed by Schmidl has been widely used among those. In this method, two identical signal intervals are made in a 1 OFDM reference symbols within a time domain, and the carrier frequency offset is estimated by using these signals.
The conventional method for estimating carrier frequency offset will be described hereinafter.
In an OFDM system, when there is carrier frequency offset, a phase of a received signal (yn) is changed with time by a carrier frequency (xcex5) in a time domain as shown in Equation 1 below.
yh=xnejxcfx80xcex5n/N xe2x80x83xe2x80x83[Equation 1]
where xn is a transmitted time-domain signal of reference OFDM symbol designed to estimate carrier frequency offset and xcex5 is a carrier frequency offset normalized to a sub-carrier spacing. N denotes the number of samples, that is, FFT size.
In the case where two identical signals are transmitted, each signal having N/2 number of samples, Equation 2 below can be obtained by correlating them.                                                                                           ∑                                      n                    =                    0                                                                              N                      /                      2                                        -                    1                                                  ⁢                                                      y                    n                    *                                    ⁢                                      y                                                                  N                        /                        2                                            +                      n                                                                                  =                                                ∑                                      N                    =                    0                                                                              N                      /                      2                                        -                    1                                                  ⁢                                                                            (                                                                        x                          n                                                ⁢                                                  ⅇ                                                      j                            ⁢                                                          xe2x80x83                                                        ⁢                            2                            ⁢                            π                            ⁢                                                          xe2x80x83                                                        ⁢                            ϵ                            ⁢                                                          xe2x80x83                                                        ⁢                                                          n                              /                              N                                                                                                                          )                                        +                                    ⁢                                      (                                                                  x                                                                              N                            /                            2                                                    +                          n                                                                    ⁢                                              ⅇ                                                  j                          ⁢                                                      xe2x80x83                                                    ⁢                          2                          ⁢                                                                                    πϵ                              ⁡                                                              (                                                                                                      N                                    /                                    2                                                                    +                                  n                                                                )                                                                                      /                            N                                                                                                                )                                                                                                                          =                                                ⅇ                                      j                    ⁢                                          xe2x80x83                                        ⁢                    π                    ⁢                                          xe2x80x83                                        ⁢                    ϵ                                                  ⁢                                                      ∑                                          n                      =                      0                                                                                      N                        /                        2                                            -                      1                                                        ⁢                                                            "LeftBracketingBar"                                              x                        n                                            "RightBracketingBar"                                        2                                                                                                          [                  Equation          ⁢                      xe2x80x83                    ⁢          2                ]            
where xn and xN/2+n (n=0, 1, 2, 3, . . . , N/2-1) are repeated identical signals in one OFDM symbol. Here * denotes a complex conjugate.
In Equation 2, as |xn|2 has a real number value, the carrier frequency offset (xcex5) can be obtained by the following Equation 3.                     ϵ        =                              1            π                    ⁢                      xe2x80x83                    ⁢                      arg            ⁡                          (                                                ∑                                      n                    =                    0                                                                              N                      /                      2                                        -                    1                                                  ⁢                                                      y                    n                    *                                    ⁢                                      y                                                                  N                        2                                            +                      π                                                                                  )                                                          [                  Equation          ⁢                      xe2x80x83                    ⁢          3                ]            
where arg is an argument function having a value from xe2x88x92xcfx80 to xcfx80. Therefore, according to the conventional method for estimating carrier frequency offset, a possible estimation range of carrier frequency offset is limited to |xcex5| less than 1.
FIG. 1 shows a block diagram of a conventional method for estimating carrier frequency offset. As shown in the drawing, in the conventional method, signals yn are received in a time domain, and a value for the signals is placed in Equation 3 to estimate carrier frequency offset.
However, it is common that the carrier frequency offset caused by oscillator instability, thermal noise in a base station and a mobile station exceeds the limited estimation range, |xcex5| less than 1. As a result, the estimation of carrier frequency offset can be failed for a large value.
The present invention has been made in an effort to solve the above problem.
It is an object of the present invention to provide a method of estimating carrier frequency offset in an OFDM system in which a range of carrier frequency offset estimation is enlarged and accuracy of the same is maintained by using only time-domain received signal.
To achieve the above object, the present invention provides a method of estimating carrier frequency offset in an OFDM system. The method includes the steps of forming a 1 OFDM symbol using an N samples, which has M repeated identical sub-blocks, form a 2i number of identical ith sub-blocks using an M/2i number of the M number of base sub-blocks as a unit; and estimating an ith carrier frequency offset xcex5i using a correlation between two adjacent ith sub-blocks in the 2i number of ith sub-blocks.
According to a feature of the present invention, the offset xcex5i of the ith carrier frequency is estimated using the following equation:             ϵ      ^        i    =                    2        xe2x80x2                    2        ⁢        π              ⁢          xe2x80x83        ⁢          arg      ⁡              (                              ∑                          n              =              0                                                      N                /                                  2                  xe2x80x2                                            -              1                                ⁢                                    y              n              *                        ⁢                          y                                                N                                      2                    xe2x80x2                                                  +                π                                                    )            
where * is a complex conjugate and yn is a received signal.
According to another feature of the present invention, M satisfies the condition of 2k(k=a positive integer).
According to yet another feature of the present invention, the method further includes the step of obtaining an enlarged ith carrier frequency offset estimate values {tilde over (xcex5)}i(qi) from the estimated ith carrier frequency offset {circumflex over (xcex5)}l, the offset estimate values {tilde over (xcex5)}i(qi) satisfying the condition of {tilde over (xcex5)}i(qi)=2iqi+{circumflex over (xcex5)}i (i=1,2,3,4 . . . ,k), where xe2x88x922kxc2x7(i+1)xe2x89xa6qi less than 2kxc2x7(i+1) when {circumflex over (xcex5)}ixe2x89xa70, and {tilde over (xcex5)}i(qi)=xe2x88x922kxc2x7(i+1)+1 xe2x89xa6qi less than 2kxc2x7(i+1)+1 when xcex5i less than 0.
According to still yet another feature of the present invention, the method further includes the steps of selecting values closest to {tilde over (xcex5)}p(qp) values among {tilde over (xcex5)}1(q1) values, where p less than k; and obtaining carrier frequency offset from the selected {tilde over (xcex5)}1(q1) values.
According to still yet another feature of the present invention, the method further includes the steps of selecting values closest to a {tilde over (xcex5)}2(q2) value among {tilde over (xcex5)}1(q1) value; selecting values closest to {tilde over (xcex5)}3(q3) values among the selected {tilde over (xcex5)}1(q1) values; selecting {tilde over (xcex5)}1(q1) value closest to {tilde over (xcex5)}p(qp) values by repeating the above step, where pxe2x89xa6k; and obtaining carrier frequency offset from the selected {tilde over (xcex5)}1(q1) value.
According to still yet another feature of the present invention, p=k, and a last remaining value among the {tilde over (xcex5)}1(q1) values is a carrier frequency offset.
According to still yet another feature of the present invention, the 1 OFDM symbol is obtained by using a data symbol Xk of a frequency scope having a random symbol that is not 0 in a sub-channel corresponding to a constant multiple of M, and having a 0 value in remaining portions.
According to still yet another feature of the present invention, the data symbol Xk of a frequency scope satisfies the condition of Xk={square root over (M)}XmD when k =mM (m=0, 1, 2, . . . , N/Mxe2x88x921), where Xk is a transmitted symbol to a kth sub-carrier, and XmD refers to a random symbol other than 0, and k is 0 with other values.